A Schmitt trigger inverter refers to a type of inverter circuit that alters the threshold voltage at which an output signal of the Schmitt trigger inverter changes state dependent upon whether the input signal to the Schmitt trigger inverter is increasing or decreasing. When the input signal is increasing and higher than a first selected threshold voltage, the output is a logic low. When the input signal is decreasing and below a second selected threshold voltage, the output is a logic high. When the input signal resides between the two selected threshold voltages, the output signal retains its state. The Schmitt trigger inverter retains its state until the input signal exceeds the appropriate threshold voltage to “trigger” a state change. The Schmitt trigger inverter is said to have memory due to the existence of the two distinct threshold voltage levels, with each threshold voltage being dependent upon the slope of the input signal. The presence of memory implies the presence of hysteresis within the Schmitt trigger inverter. Hysteresis generally refers to the dependence of the current state of a system on the history of the system.
Schmitt trigger inverter architectures are typically used to improve the noise and/or the distortion immunity of an inverter circuit. Conventional inverters switch as their input signal crosses a single threshold voltage regardless of the previous state of the inverter. When the input signal to a conventional inverter resides close to the singular threshold voltage, any noise appearing within the input signal can cause the voltage of the input signal to transition back and forth across the threshold voltage. For example, as large blocks of digital circuits within an IC are switched, a noise signal associated with the switching can propagate within the IC and appear at the input to, or within, a supply voltage powering the inverter. The noise within the input signal can result in unintended changes in the output state of the inverter.
The presence of hysteresis within the Schmitt trigger inverter can prevent the output state of the Schmitt trigger inverter from being changed by noise so long as the noise resides within the voltage range between the two threshold voltages selected for the Schmitt trigger inverter. In this manner, noise is prevented from altering the output state of the Schmitt trigger inverter or, in a worst case, causing the Schmitt trigger inverter to oscillate. As such, Schmitt trigger inverters are frequently used within ICs generating, or operating within, high noise environments.